1. Field of the Invention
The present invention pertains to a system employed for determining the global position of a mobile unit by employment of both an inertial reference system (IRS) and a satellite positioning system (GPS), and more specifically, a system which employs a provision for determining the mobile unit's global position and corresponding integrity during those periods of time in which the GPS satellite constellation is insufficient for establishing GPS integrity limit values by employment of RAIM.
2. Description of the Related Art
Satellite positioning systems are now well-known in the art. Such systems, for example, NAVSTAR-GPS, are rapidly being employed for a determination of the geocentric position of mobile units, such as water and land vehicles, space and aircraft, and survey equipment, to name a few.
In aircraft, GPS systems are being utilized for navigation, flight control, and airspace control. These GPS systems may operate independently or in combination with inertial reference systems or attitude heading reference systems in order to provide information particularly during a flight mission.
Global positioning systems, hereinafter referred to as "GPS", similar to NAVSTAR, commonly use a GPS receiver, located on a mobile unit, for receiving satellite information signals transmitted from a plurality of satellites. Each GPS satellite transmits a satellite information signal containing data that allows a user to determine the range or distance between selected GPS satellites and the antenna associated with the mobile unit's GPS receiver. These distances are then used to compute the geocentric position coordinates of the receiver unit using known triangulation techniques. The computed geocentric position coordinates may, in turn, be translated to earth latitude and longitude coordinates.
In order to determine the position of the GPS receiver, a minimum of four unique satellite information signals are required, rather than the expected three (three position, unknown coordinates). This is so, since the GPS receiver generally includes a receiver clock which is not as accurate as the atomic clock normally associated with each of the satellites. Therefore, receiving satellite information signals from four different satellites provides a complete solution which permits the correction of any receiver clock error as is well-understood in the art. Herein, the GPS receiver position derived by the triangulation technique using data from multiple satellites is referred to as the "GPS estimated position", identified as POS.sub.-- GPS. The accuracy of this estimated GPS position is dependent upon many factors, including, among others, atmospheric conditions, selective satellite availability, and the relevant position of the satellites with respect to the line of sight view of the satellites.
Associated with a GPS estimated position is a "position error bound" as particularly defined by accepted GPS systems standards which have been developed by the Radio Technical Commission for Aeronautics (RTCA), in association with aeronautical organizations of the United States from both government and industry. The RTCA has defined the phrase "GPS system integrity" as the ability of a GPS system to provide timely warnings to users when the GPS system should not be used for navigation. "System integrity" is particularly identified in a document entitled "Minimum Operational Performance Standards for Airborne Supplemental Navigation Equipment Using Global Positioning System (GPS)", document number RTCA/DO-208, July 1991, prepared by: SC-159, beginning at section 1.5. As described therein, GPS is complicated in that it is a four-dimensional system involving three components of position and one time component. As also described in the aforesaid RTCA publication, the signal-in-space error transforms into a horizontal position error via a relatively complex function of a satellite constellation geometry at any given moment. The GPS integrity system must interpret the information it has about the received GPS signals and error terms in terms of the induced horizontal position error, commonly referred to as the "position error bound", and then make a decision as to whether the position error bound is outside the allowable radial error, specified for a particular phase of the flight mission in progress. The allowable error is referred to as the "alarm limit", herein referred to as the "integrity alarm limit". If the horizontal position error bound is found to exceed the integrity alarm limit, a timely warning must be issued by the GPS receiver or subsystem to notify the pilot that the GPS estimated position should not be relied upon.
Two rather distinct methods of assuring GPS integrity have evolved as civilian use of GPS has progressed. One is the Receiver Autonomous Integrity Monitoring (RAIM) concept, and the other is the ground monitoring approach that goes under the "GPS Integrity Channel" (GIC). The intent of both of these methods is the calculation of the position error bound with regard to the current GPS estimated position so that it may be compared with the alarm limit associated with a particular phase of a flight mission.
The receiver autonomous integrity monitoring system (RAIM) employs a self-consistency check among the measurements, more specifically, GPS pseudo range measurements. Satellite redundancy is required to perform a self-consistency check on an instantaneous basis. Thus, five satellites must be in view, i.e., five satellite information signals received and pseudo range measurements calculated by a GPS receiver. If fewer than five satellites are in view, the value of the predicted position error bound will be infinite. Also, constraints are placed on the satellite constellation geometry that must be met if the self-consistency check is to be effective in the presence of noise, e.g., azimuth angle of the satellite relative to user position. Generally, a satellite constellation with many satellites in view permits a robust integrity monitoring system. Conversely, a satellite constellation having only a few satellites in view, may limit the availability of an integrity monitoring system. Thus, there may be short periods when a good consistency check is not possible (less than five satellites in view). The main feature of RAIM is that it is completely self-contained and relatively easy to implement in software.
Examples of RAIM may be found in the aforementioned RTCA publication, Appendix F, and also in an article entitled "Implementation of a RAIM Monitor and a GPS Receiver and an Integrated GPS/IRS" by Mats Brenner, located at page 397, in the proceedings of ION GPS-90, Third International Technical Meeting of the Satellite Division of the Institute of Navigation, Sep. 19-21, 1990.
GPS systems which incorporate RAIM output a position error bound value which represents the probabilistic radial errors of the navigation solution, namely, the GPS estimated position of the receiver unit. Currently, RAIM may generate several numbers, including, a horizontal position error bound value (sometimes referred to as HIL--Horizontal Integrity Limit), a vertical position error bound value (sometimes referred to as VIL--Vertical Integrity Limit), and spherical position error bound for the current time, i.e., the instance of time that GPS measurements occurred.
Once calculated, the position error bound value(s), HIL and/or VIL, may be compared with selectable integrity alarm limit values to determine if the pilot can rely on the derived GPS estimated position for the current phase of the light mission. It should be recognized that some interpretation may be required dependent upon the GPS receiver's ability to simultaneously receive a plurality of satellite information signals as is well-understood in the art. However, advancements in the art of 12-channel GPS receivers have made it no longer necessary to rely on interpolation of data as before.
The allowable integrity alarm limit values may change depending upon the phase of the flight mission. For instance, if a pilot is flying in the terminal phase, the integrity alarm limit may be less stringent than if the pilot is in the approach phase of the flight mission. If the pilot is to transition from the terminal phase to the approach phase, the pilot needs to know whether the current position error bound is sufficient to allow the pilot to rely upon the GPS solution to make the transition.
As is well understood in the art, inertial reference systems employ a plurality of inertial sensors, for example, gyroscopes and accelerometers, for determining an IRS estimated position of the aircraft, hereinafter referred to as "POS.sub.-- IRS". Generally, the IRS estimated position is in terms of latitude and longitude (altitude being separately determined by other means such as an altimeter of some type). However, inherent in such inertial sensors are particular bias and drift terms which affect the accuracy of the IRS estimated position of the aircraft utilizing solely an inertial reference system. Since high inertial grade sensors, i.e., low bias and drift characteristics, are very costly, it is desirable to minimize the cost of the IRS system by using lower grade inertial sensors.
In the art, a compromise has been reached by using lower grade inertial reference systems in combination with a global positioning system to produce a high quality--lower cost navigation and flight control system. This is sometimes referred to as a Hybrid INS/GPS or IRS/GPS Inertial Reference System. These systems achieve excellent results since low grade inertial reference systems produce very accurate dynamic response characteristics, whereas, GPS provides very accurate static position information, but less accurate dynamic response information. Combining both the IRS estimated position and inertial reference information with GPS estimated position information provides excellent user position information for flight navigation and flight control applications. Accordingly, a flight management system (FMS), combines the excellent features of both the IRS and the GPS systems to provide position and inertial reference information which permits excellent flight management, flight control and navigation.
An example of a hybrid IRS/GPS system is Honeywell Inc.'s "Global Positioning Inertial Reference Unit (GPIRU) identified as an HG 1050 AG01 which is referred to as a "hybrid" system since it provides position and inertial information which are a resultant combination of GPS and inertial reference system information. The GPIRU includes an inertial reference unit with gyros and accelerometers to provide information about aircraft attitude and rate of change of position as well as providing a first source of position information. The GPIRU also receives inputs from a Global Position System Receiver to provide a second and independent source of information about the aircraft position. The two sets of information are mathematically combined in a Flight Management System (FMS) to determine a hybrid position POS.sub.-- HYB. In turn, this position value along with attitude and rate signals from the Inertial Reference Unit may be provided in a flight control for controlling aircraft.
A problem, however, with flight management systems employing GPS and IRS is the questionable integrity of the GPS estimated position information during those times in which RAIM integrity limit values are no longer available, i.e. insufficient satellite information to provide useful integrity position error bound values.